[codex] Add BytePlus coding-plan variant to setup wizard

[Maaannnn]

Summary

Add BytePlus as a built-in API-key provider in metaclaw setup and model Volcengine/BytePlus coding-plan support as a plan_variant instead of separate top-level providers.

What Changed

• add byteplus immediately after volcengine in the setup wizard provider list
• prompt for plan_variant (standard or coding-plan) when the user selects volcengine or byteplus
• save llm.plan_variant in config and show it in config summaries
• preserve existing proxy config fields when re-running the setup wizard
• update the English and Chinese README setup/config docs to reflect the new provider contract

User Impact

Users can now choose BytePlus directly in metaclaw setup, and both Volcengine and BytePlus expose a built-in coding-plan variant without expanding the provider list into separate *-coding-plan entries.

Validation

• python3 -m py_compile metaclaw/setup_wizard.py metaclaw/config_store.py metaclaw/config.py tests/test_setup_wizard.py
• smoke check for provider preset resolution and plan variant inference
• in-memory setup wizard smoke check for byteplus + coding-plan

Gaps

• python3 -m pytest tests/test_setup_wizard.py tests/test_utils.py could not run in this environment because pytest is not installed
• other translated README files were not updated in this PR